ECE 8830 Electric Drives

1
ECE 8830 - Electric Drives
Topic 16 Control of SPM Synchronous
Motor Drives Spring 2004
2
Introduction

• Control techniques for synchronous motor
  drives are similar to those for induction motor
  drives. We will consider both scalar and vector
  control for surface PM motor (both sinusoidal
  and trapezoidal PM motor) drives, reluctance
  motor drives, and wound field synchronous motor
  drives.

3
Sinusoidal SPM Motor Drives

• The ideal synchronous motor torque-speed
  characteristic at a single frequency excitation
  is as shown below

Torque
Motoring Mode
0
Speed
Generating Mode
4
Sinusoidal SPM Motor Drives (contd)

• Thus, the motor either runs at synchronous
  speed or doesnt run at all. Two control
  approaches - open loop V/Hz control and
  self-control mode.
• In open-loop V/Hz control, the frequency of
  the drive signal is used to control the
  synchronous speed of the motor.
• In self-control, feedback from a shaft encoder
  is used to effect the control.

5
Sinusoidal SPM Motor Drives (contd)

• Open-loop V/Hz control is the simplest control
  approach and is useful when several motors need
  to be driven together in synchrony. Here the
  voltage is adjusted in proportion to the
  frequency to ensure constant stator flux, ?s. An
  implementation of this control strategy is shown
  on the next slide.

6
Sinusoidal SPM Motor Drives (contd)

7
Sinusoidal SPM Motor Drives (contd)

• The control characteristics are shown in the
  figure below

8
Sinusoidal SPM Motor Drives (contd)

• Neglecting the stator resistance, and using
  the field flux ?f as the reference phasor, a
  phasor diagram of the synchronous motor is shown
  below

9
Sinusoidal SPM Motor Drives (contd)

• The torque developed by the motor is given by
• where Iscos? is the in-phase component of the
  stator current and ? is the torque angle.
• If ?e is changed too quickly, the system will
  become unstable. The max. rate of
  acceleration/deceleration is given by
• where Ter rated torque and TL load torque.

10
Sinusoidal SPM Motor Drives (contd)

• A self-controlled scheme for an SPM motor is
  shown below
• Here the frequency and phase of the inverter
  output are controlled by the absolute position
  encoder mounted on the motor shaft.

11
Sinusoidal SPM Motor Drives (contd)

• The absolute position encoder required for
  self-controlled drives for synchronous motors are
  one of two types - an optical encoder or a
  mechanical resolver with decoder.

12
Sinusoidal SPM Motor Drives (contd)

• An optical encoder has alternating opaque and
  transparent segments. A LED is placed on one side
  and a photo-transistor on the other side. A
  binary coded disk is shown below
• With 14 rings (14-bit resolution) a resolution
  of 0.04 electrical degrees can be achieved for a
  four-pole motor with this type of encoder.

13
Sinusoidal SPM Motor Drives (contd)

• Another type of optical encoder is the slotted
  disk optical encoder. The below encoder is
  specifically designed for a 4-pole motor.

14
Sinusoidal SPM Motor Drives (contd)

• There are many slots in the outer perimeter
  and two slots 180? apart on the inner radius.
  There are four optical sensors S1-S4. S4 is
  located on the outer perimeter and the S1-S3
  sensors are located 60 apart on the inner
  radius. The sensor outputs are as shown below

15
Sinusoidal SPM Motor Drives (contd)

• The block diagram of a resolver with decoder is
  shown below

16
Sinusoidal SPM Motor Drives (contd)

• The analog resolver is basically a 2? machine
  that is excited by a rotor-mounted field winding.
  The primary winding of a revolving transformer is
  excited by an oscillator with voltage VV0sin?t.
  The stator windings of the resolver generate
  amplitude-modulated output voltages
• and

17
Sinusoidal SPM Motor Drives (contd)

• The decoder converts the analog voltage
  outputs to digital position information. The
  high-precision sin/cos multiplier multiplies V1
  and V2 by cos and sin respectively. An error
  amplifier takes the difference of these two
  output signals to generate the signal
  . The phase sensitive demodulator creates a
  dc output that is proportional to .
  An integral controller, VCO, and up-down counter
  together generate an estimated . Under steady
  state conditions the tracking error will be zero.
  

18
Sinusoidal SPM Motor Drives (contd)

• Vector control of a sinusoidal SPM motor is
  relatively simple. Because of the large effective
  airgap in this type of motor, the armature flux
  is very small so that ?s ? ?m ? ?f . For maximum
• torque sensitivity (and
• therefore efficiency) we
• set ids0 and iqs.

19
Sinusoidal SPM Motor Drives (contd)

• The torque developed by the motor can be
  expressed as
• where is the space vector magnitude
• ( ) and .
• A block diagram of a vector control
  implementation for a sinusoidal SPM motor is
  shown in the next slide.

20
Sinusoidal SPM Motor Drives (contd)

• Note This vector control scheme is only valid
  in the constant torque region.

21
Sinusoidal SPM Motor Drives (contd)

• The upper limits of the available dc-link
  voltage and current rating of the inverter limit
  the maximum speed available at rated torque to
  the base speed (?b). However, it is often
  desirable to operate at higher speeds (e.g. in
  electric vehicles). Above base speed, however,
  the induced emf will exceed the input voltage and
  so current cannot be fed into the motor. By
  reducing the induced emf, by weakening the air
  gap flux linkages, higher speeds can be obtained.
  

22
Sinusoidal SPM Motor Drives (contd)

• In order to achieve the field weakening, a
  demagnetizing current -ids must be injected on
  the stator side. However, this ids must be large
  because of low armature reaction flux, ?a. This
  small weakening of ?s results in a small range of
  field-weakening speed control.
• Let us consider next how to extend the vector
  control scheme to speeds beyond base speed (?b),
  i.e. into the field-weakening region.

23
Sinusoidal SPM Motor Drives (contd)

• A phasor diagram for field-weakening control
  is shown below

24
Sinusoidal SPM Motor Drives (contd)

• The injected -ids which provides the flux
  weakening results in a rotation of the vector. At
  a, which corresponds to zero torque
  and maximum speed, ?r1. At this condition, ?0,
  ?s?s, VfVf and VsVs (see phasor diagram).
• The field weakening region can be increased by
  increasing the stator inductance (see
  torque-speed diagram on next slide).

25
Sinusoidal SPM Motor Drives (contd)

26
Sinusoidal SPM Motor Drives (contd)

• A block diagram of a vector control drive for
  a sinusoidal SPM motor including the field
  weakening region is shown below

27
Sinusoidal SPM Motor Drives (contd)

• In constant torque mode, ids0 but in
  field-weakening mode, flux is controlled
  inversely with speed with -ids control generated
  by the flux loop.
• Within the torque loop, iqs is controlled to
  be limited to the value,
• where is the rated stator current.

28
Control of Brushless DC Motor Drives

• Trapezoidal synchronous permanent magnet
  motors have performance characteristics
  resembling those of dc motors and are therefore
  often referred to as brushless dc motors (BLDM).
• Concentrated, full-pitch stator windings in
  these motors are used to induce 3? trapezoidal
  voltage waves at the motor terminals. Thus a 3?
  inverter is required to drive these motors as
  shown in the next slide.

29
Control of BLDM Drives (contd)

• The inverter can operate in two modes
• 1) 2?/3 angle switch-on mode
• 2) Voltage and current control PWM mode

30
Control of BLDM Drives (contd)

• The 2?/3 angle switch-on mode is shown in the
  below figure

31
Control of BLDM Drives (contd)

• The switches Q1-Q6 are switched on so that the
  input dc current Id is symmetrically located at
  the center of each phase voltage wave. At any
  instant in time, one switch from the upper group
  (Q1,Q3,Q5) and one switch from the lower group
  (Q2,Q4,Q6) are on together. The absolute position
  sensor is used to ensure the correct timing of
  the switching/commutation of the devices. At any
  time, two phase CEMFs (2Vc) of the motor are
  connected in series across the inverter input.
  ?The power into the motor is 2VCId.

32
Control of BLDM Drives (contd)

• In addition to controlling commutation by the
  timing of the switches in the PWM inverter, it is
  also possible to control the current and voltage
  output of the inverter by operating the PWM in a
  chopper mode. This is the voltage and current
  control PWM mode of operation of the drive.

33
Control of BLDM Drives (contd)

• The average output current and voltage are set
  by the duty cycle of the switches in the PWM
  inverter. Varying the duty cycle results in
  variable average output current/voltage.
• Two chopping modes can be used - feedback mode
  and freewheeling mode.
• In feedback mode, two switches are switched on
  and off together (e.g. Q1 and Q6) whereas in
  freewheeling mode, the chopping is performed only
  on one switch at a time.

34
Control of BLDM Drives (contd)

35
Control of BLDM Drives (contd)

• Consider the feedback mode with Q1 and Q6 as
  the controlling switching devices. During the
  time that these switches are on, the phase a and
  b currents are increasing but during the time
  that they are off, the currents will decrease
  through feedback through the diodes D3 and D4.
  The average terminal voltage Vav will be
  determined by the duty cycle of the switches.

36
Control of BLDM Drives (contd)

• Now consider the freewheeling mode of
  operation. When Q6 is on Vd is applied across ab
  and the current increases. When Q6 is turned off,
  freewheeling current flows through Q1 and D3
  (effectively short-circuiting the motor
  terminals) and the current decreases (due to the
  back emf).

37
Control of BLDM Drives (contd)

• The steady state torque-speed characteristics
  for a brushless dc motor can be easily derived.
  Ignoring power losses, the input power is given
  by
• The torque developed by the motor is simply,

38
Control of BLDM Drives (contd)

• The back emf is proportional to rotor speed and
  is given by
• where K is the back emf constant and ?r is the
  mechanical rotor speed (P/2) ?e. The steady
  state (dc) circuit equation for any switch
  combination is

39
Control of BLDM Drives (contd)

• The torque expression can be rewritten as
• where P of motor poles. If we define the
  base torque as
• where Isc is the short-circuit current given
  by
• gt

40
Control of BLDM Drives (contd)

• The rotor base speed ?rb can be defined as
• The torque-speed relationship can be derived by
  combining these equations, yielding
• gt
  Te(pu)1-?r(pu)
• where Te(pu)Te/Teb and ?r(pu) ?r/ ?rb

41
Control of BLDM Drives (contd)

• This normalized torque-speed relation is
  plotted below. Note the droop in the no-load
  speed due to the stator resistance voltage drop.

42
Control of BLDM Drives (contd)

• A closed loop speed control system for a BLDM
  drive with a feedback mode operation of the PWM
  inverter is shown below

43
Control of BLDM Drives (contd)

• Three Hall effect sensors are used to provide
  the rotor pole position feedback. This gives
  three 2?/3-angle phase shifted square waves (in
  phase with the phase voltage waves). The six step
  current waveforms are then generated by a
  decoder.
• The speed control loop generates Id from the
  ?r command speed. The actual command phase
  currents are then generated by the decoder.
  Hysteresis current control is used to control the
  phase currents to track the command phase
  currents.

44
Control of BLDM Drives (contd)

• A freewheeling mode close loop current drive
  for a BLDM is shown below

45
Control of BLDM Drives (contd)

• In this case the three upper devices (Q1, Q3,
  and Q5) are turned on sequentially in the middle
  of the positive half-cycles of the phase voltages
  and the lower devices (Q2,Q4 and Q6) are chopped
  sequentially in the middle of the negative
  half-cycles of the phase voltages to achieve the
  desired current Id. This is all timed through
  the use of the Hall sensors and the decoder logic
  circuitry. One dc current sensor (R connected to
  ground) is used to monitor all three phase
  currents.

46
Control of BLDM Drives (contd)

• The controller section and power converter
  switches outlined by the dotted line can be
  integrated into a low-cost power integrated
  circuit. An example of a commercial BLDM
  controller IC is the Apex Microtechnology BC20
  (see separate handout).

47
Control of BLDM Drives (contd)

• Pulsating torque can be a problem with BLDM
  motors (see figure below).

48
Control of BLDM Drives (contd)

• The high frequency component is due to ripple
  current from the inverter and is filtered out by
  the motor. The rounding of the torque is due to
  the rounding of the phase voltages (caused by
  leakage flux adjacent to the magnet poles) and
  this generates significant 6th harmonic torque
  pulsation. A higher number of poles in the
  machine can help to alleviate this problem.

49
Control of BLDM Drives (contd)

• The speed range of a BLDM motor can be
  extended beyond the base speed range (just as in
  the case of the sinusoidal SPM motor). This can
  be achieved by advancing the angle ? which is
  used to locate the position of the current
  waveforms with respect to the phase voltage
  waveforms (?0 locates the current waveforms in
  the center of the voltage waveforms). Also, if we
  change from a 2?/3 conduction mode to a ?
  conduction mode.

50
Control of BLDM Drives (contd)

• The normalized torque-speed curve for extended
  range is shown below for different ? angles for
  2?/3 conduction mode (solid lines) and ?
  conduction mode (dotted lines).

51
Simulation of PM Synchronous Motor Drives

• Project 5 at the end of Ch. 10 Ong provides a
  study of a self-controlled permanent magnet
  synchronous motor. The motor parameters for the
  70 hp, 4-pole PM motor are given in the table
  below

52
Simulation of PM Synchronous Motor Drives
(contd)

• The steady state equations used in the
  simulation are given in the following table

53
Simulation of PM Synchronous Motor Drives
(contd)

• If we assume that the output torque varies
  linearly with stator current Is, the torque
  expression can be rewritten as
• This is a nonlinear equation with a single
  unknown, ?. Once ? is found, the current and
  voltage components in the q and d rotor reference
  frame and the power factor angles and the
  stationary q, d current components can be
  calculated.

54
Simulation of PM Synchronous Motor Drives
(contd)

• The steady state curves are shown below

55
Simulation of PM Synchronous Motor Drives
(contd)

• Some observations from these curves
• Output torque ? Iqe and Iq (almost linear) thus
  torque control can be accomplished by controlling
  Iqe (or Iq) with Id controlled as shown.
• The power factor angles
• 1/2 torque angle. This
• results in the phasor
• diagram shown (for the
• motoring mode)

56
Simulation of PM Synchronous Motor Drives
(contd)

• A Simulink simulation model for a
  self-controlled PM drive is shown below

57
Simulation of PM Synchronous Motor Drives
(contd)

• Some points regarding this simulation model
• iq and id are used to control the output torque.
• Torque command is implemented using a repeating
  sequence source.
• A rate limiter is used to limit the reference
  torque input to the torque controller.
• The inner id and iq control loops are closed
  loops.

58
Simulation of PM Synchronous Motor Drives
(contd)

• The feedback block uses the stator phase currents
  and rotor position to generate id and iq.
• The coordinated reference values for id and Vs
  are generated by separate function generator
  blocks (Id-Iq and Vs-Tem, respectively)
  implemented using a curve fit to the steady state
  data shown earlier.
• Dynamic simulation results are shown on the
  next slide.

59
Simulation of PM Synchronous Motor Drives
(contd)